Renewable energy power generators, which generate power using clean, renewable energy, have been widespread to respond to the recent attention to global environmental issues. Such renewable energy power generators can have greatly varying outputs due to changes in weather. To reduce such output variations, systems combining a renewable energy power generator and a storage battery have been developed.
Such systems now focus on maintaining the charged level using the storage battery and reducing any sudden change in their output from the entire system. Similar attempts also use a cogeneration system or a fuel cell with a storage battery under such control. A system combining a power generator, such as a renewable energy power generator, a cogeneration system, or a fuel cell, with a storage battery described above is also referred to as a distributed power system.
A typical example of such distributed power systems is a photovoltaic system with a storage battery. The photovoltaic system connects a solar module, which is a direct current power source, to the utility grid using a power conditioner. The power conditioner usually includes a direct current-to-direct current (DC-DC) converter to which a solar module is connected, a bidirectional direct current-to-alternating current (DC-AC) inverter connected to the utility grid, and a bidirectional DC-DC converter having its high-voltage end connected to a node between the DC-DC converter and the DC-AC inverter and its low-voltage end connected to the storage battery.
This bidirectional DC-DC converter has the high-voltage end receiving and outputting high-voltage direct current power from or to the DC-DC converter to which a solar module is connected, and has the low-voltage end connected to the low-voltage storage battery. The high-voltage end DC power is converted first to AC power by a switching circuit. The voltage of the AC power is then converted by a high-frequency transformer. The resultant power is then converted back to DC power by the switching circuit. This enables bidirectional DC-DC conversion between the high and low voltage ends. The DC-DC converter can charge the storage battery with power from the solar module and can discharge the power stored in the storage battery to a load.
The above bidirectional DC-DC converter includes the switching circuit to convert power between the high and low voltage ends. The on or off operation of a switching element included in the switching circuit may generate a reverse recovery current or a surge voltage. A reverse recovery current or a surge voltage can overload the switching element instantaneously and break the switching element. An active snubber may be used to reduce the influence of a reverse recovery current or a surge voltage and to prevent the power efficiency of the circuit from decreasing. The active snubber temporarily charges a capacitor with a surge current and discharges the surge current in addition to a transfer current during power transfer to regenerate the energy.
One example using a known active snubber may be a secondary converter circuit including a pair of switching elements including an antiparallel diode and a parallel capacitor and connected to a secondary end of a high-frequency transformer, an energy storage element connected to a center tap of the high-frequency transformer, and a voltage clamp circuit arranged between the high-frequency transformer and the switching elements of the secondary converter circuit. The voltage clamp circuit includes a pair of series circuits each including a capacitor and a switching element having an antiparallel diode, which are connected in parallel with their polarities being reversed (refer to, for example, Patent Literature 1).
Another known example is a low-voltage switching circuit including a group of first switching elements, a smoothing reactor, a second smoothing capacitor, and a voltage clamp circuit. The group of first switching elements is connected to a second direct current power source and a secondary winding of a transformer. The smoothing reactor is connected to the group of first switching elements and/or the secondary winding of the transformer. The second smoothing capacitor has one end connected to one end of a switching element of the group of first switching elements, has the other end connected to one end of the smoothing reactor, and is connected in parallel to the second direct current power source. The voltage clamp circuit is connected to a switching element of the group of first switching elements, and includes a clamp capacitor and a group of second switching elements including at least one switching element (refer to, for example, Patent Literature 2).
However, the first technique described above uses multiple capacitors in the active snubber (voltage clamp circuit). This may complicate the system configuration, and cannot decrease the cost. The second technique uses a clamp capacitor in the active snubber (voltage clamp circuit) arranged between the negative end of the second direct current power source and the group of second switching elements, increasing the voltage applied to the clamp capacitor. This increases the breakdown voltage and the capacity of the clamp capacitor, and cannot decrease the cost.